What is the “NoUserAgent_HEADER” Rule in AWS WAF? A Detailed Explanation of Its Function, Behavior, and Countermeasures
Introduction: Basics of AWS WAF and NoUserAgent_HEADER
AWS WAF (Web Application Firewall) is a managed service that protects web applications from attacks. Among its managed rules (Core Rule Set), “NoUserAgent_HEADER” is one such security rule provided by AWS.
This rule blocks HTTP requests that do not include the “User-Agent” header. In this article, we will explain in detail how this rule works, how to use it, and what precautions to take.
1. What is NoUserAgent_HEADER?
The “User-Agent” header in an HTTP request is an important element that indicates the client sending the request (e.g., browser, bot, application). While normal web browsers always include it, some unauthorized accesses or bot attacks may deliberately omit this header.
The AWS WAF “NoUserAgent_HEADER” rule detects such requests and blocks them, considering them a security risk.
Main Detection Criteria
- Requests without a User-Agent header
- Empty User-Agent values (with some exceptions)
2. Why is This Rule Necessary?
Requests without a User-Agent can pose the following risks:
- Attackers using bots or scripts to attempt unauthorized access
- Crawlers targeting unauthenticated API endpoints
- Probing requests from malicious scanning tools
In other words, this rule serves as a first line of defense against suspicious anonymous requests.
3. Problems and Countermeasures in Practice
❗ Example Issues
- Some API integrations or webhooks (e.g., CI tools, IoT devices, Cognito, ShareFile) may not send a User-Agent, resulting in unintentional blocks.
- Communication via third-party tools or curl without a User-Agent may result in a 403 Forbidden response.
✅ Countermeasures
1. Add Conditional Exception Rules
Disable or set the NoUserAgent_HEADER rule to Count mode for specific paths (e.g., /api/health, /oauth/token) to log the requests instead of blocking them.
2. Explicitly Add User-Agent
Add a User-Agent explicitly for curl or script-based communications like so:
curl -A "MyClient/1.0" https://example.com/api
3. Monitor and Visualize WAF Logs
Use CloudWatch Logs or AWS WAF’s labeling feature to visualize how often the NoUserAgent_HEADER rule blocks requests, and adjust configurations accordingly.
4. Features of Managed Rules and Tips for Selection
The NoUserAgent_HEADER rule in AWS WAF’s Core Rule Set has the following characteristics:
| Item | Description |
|---|---|
| Target | All HTTP requests |
| Condition | User-Agent is missing or empty |
| Response | Blocks by default (can be set to Count mode) |
| Purpose | Control access from anonymous bots and malicious scripts |
When deploying, start with Count mode to observe behavior, then apply or exclude the rule to specific endpoints as needed to balance security and usability.
Intended Audience and Accessibility Consideration
This article is mainly for infrastructure engineers, DevOps personnel, and security professionals responsible for web security in AWS environments. Technical terms are explained in plain language, and differences in configurations and behavior are visually illustrated with tables and code examples. This makes the content accessible to readers with visual impairments. By avoiding reliance on colors and images, we ensure a high level of accessibility.
Summary
- NoUserAgent_HEADER is an AWS WAF security rule that blocks requests lacking a User-Agent header.
- While effective against unauthorized access, it may also block legitimate requests, so careful application and exception settings are crucial.
- By utilizing Count mode and explicitly setting User-Agent headers, you can maintain both security and functionality.
Leverage the advanced features of AWS WAF to build secure and flexible web security.